If, on the first observation, the system is in state 2, what is the probability that it alternates between states 1 and 2 for the first four...

If, on the first observation, the system is in state 2, what is the probability that it alternates between states 1 and 2 for the first four observations (i.e., it occupies state 2, then state 1, then state 2, and finally state 1 again)?